Animals cannot manufacture certain substances, and they have to obtain them from food. These substances are called essential nutrients. As you might expect, essential nutrients differ from species to species, and they include essential amino acids, essential fatty acids, vitamins, and minerals.

Essential fatty acids are needed for many body processes, including maintaining healthy skin, building cell membranes, and synthesizing important biologically active compounds.

Essential fatty acids include linoleic acid, a compound needed to make some of the phospholipids used for membrane formation. Fatty acid deficiencies are rare because most diets include sufficient quantities of these nutrients.

We need a total of eight essential amino acids in our diet. These essential amino acids include leucine, valine, and tryptophan. You don’t need a lot of different foods in your diet to get these essential nutrients. All eight are found in a meal of corn and beans!

Our body can synthesize the other twelve amino acids we need. These amino acids include argenine, tyrosine, and glycine.

Amino acids are used to synthesize proteins, and if one or more essential amino acids are missing from the diet, a protein deficiency may result. As different proteins require different combinations of amino acids, amino acid deficiencies may affect a number of different proteins and may produce a variety of symptoms.

A deficiency of the amino acid leucine causes the disease called kwashiorkor, common in some parts of Africa. Symptoms include depression and acute hunger, and people with kwashiorkor have swelling, loss of muscle mass, fatigue, and decreased immunity.

Tryptophan is a precursor to the hormone serotonin, which plays a key role in the transmission of nerve impulses in the body. Tryptophan deficiency can lead to depression, insomnia, and chronic fatigue. Some people claim that adding tryptophan to the diet is effective in treating a variety of neurological disorders.

All elements that are required by an animal are by definition essential nutrients. An element can’t be synthesized from substances that don’t contain that element—unless you have a nuclear reactor in your digestive system!

Some mineral elements, like sodium, potassium, calcium, and iron, are needed in fairly large amounts: between 10 milligrams per day for iron, and at least 1000 milligrams per day for calcium. Calcium is one of the most important elements in the body. Our bones are largely constructed of calcium salts. If the level of calcium in the blood is not maintained at a high enough level, bone will be broken down to maintain the blood calcium level. A deficiency of calcium in the body can therefore lead to osteoporosis, a condition where the bone is no longer as solid and strong as it should be.

Others minerals, called trace elements, are essential for health but in far smaller amounts than iron or calcium. For example, adults need up to 0.2 milligrams each day of chromium, an element important in regulation of blood sugar.

The existence of vitamins was suspected late in the nineteenth century, when it was found that a diet of pure carbohydrates, fats, and proteins would not support life.

Vitamins are organic compounds that the body requires in very small quantities—up to 100 milligrams daily. Thirteen different vitamins have been identified as essential to humans, and a variety of foods are rich in one or more vitamins. The sale of vitamin supplements has become a multibillion dollar industry in the United States. However, most people who eat a balanced diet of vitamin-rich foods are not likely to need supplements.

As you may know, vitamin C, or ascorbic acid, was one of the first vitamins to be discovered. On long voyages, sailors on ships usually lived on a diet that was low in fresh fruit and vegetables. This diet resulted in a vitamin-deficiency disease called scurvy. British sea captains learned to combat scurvy by carrying a supply of limes as a source of vitamin C.

Ascorbic acid is an essential human vitamin, but to a rabbit, it's just another chemical. Rabbits and some other animals have bacteria in their intestines that can manufacture ascorbic acid from other nutrients.

Vitamins are categorized as fat-soluble vitamins, which can be stored in the body, and water-soluble vitamins, which must be continually ingested. Any excess water-soluble vitamins are excreted in the urine.

Vitamins were originally placed in categories based on their function in the body, and they were given letter names. Later, as their chemical structures were revealed, they were also given chemical names. Today, both naming conventions are used.

Fat-soluble vitamins include vitamins A, D, E, and K. Water-soluble vitamins include vitamins B and C, as well as biotin, pantothenic acid, and folacin.

There are four B vitamins, called the B complex vitamins, that function as coenzymes in important metabolic processes.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education